amphoe.com is one of the very good sources on information of the districts in Thailand, though in some cases there are smaller flaws in the details. Searching through the archived news reports of the National News Bureau of Thailand I now found a small reference on how the information on the website came into being.
Sheriffs have been ordered to update information on district websites nationwide from February 18 2005 reveals that it were the district officers (here strangely translated as sheriffs) who were ordered to provide the data to include on the website. This also explains the fact that the quality and details of the entries differ significantly, as some officers just did the necessary while others showed a lot of initiative.
It also shows that the website is in fact a project by the Department of Provincial Administration (DOPA), and that only one year passed after the deadline of the above request till the Wikipedia project on the districts got pace based on this contents.